1. Field of the Invention
The present invention relates to a semiconductor device of a multi-wiring structure, particularly, to the structure of an insulating film within the same wiring layer.
2. Description of the Related Art
In order to improve the performance of a semiconductor device, particularly, an LSI, the dielectric constant of the insulating film used in the multi-wiring structure is being made lower and lower. To be more specific, by lowering the dielectric constant of the insulating film formed between adjacent wiring layers, the parasitic capacitance between the adjacent wiring layers is lowered, and the delay time of the signal propagated through the wiring is improved so as to improve the operation speed of the LSI.
In order to lower the dielectric constant k of the insulating film to 3 or less, it is necessary to lower the density of the insulating film. However, because of the trade off relationship with the mechanical strength of the insulating film, the mechanical strength of the insulating film is rendered insufficient with decrease in the dielectric constant of the insulating film.
In the semiconductor device of the conventional multi-wiring structure, the insulating film used in the same wiring layer is formed of a single material. If the insulating film is formed of a material having a low dielectric constant, problems are generated as follows.
First of all, in the case of using an insulating film having a low dielectric constant in the multi-wiring structure, the insulating film is incapable of withstanding the mechanical impact in, for example, the bonding step and the packaging step, leading to breakage of the insulating film.
For example, FIG. 1 shows the case where the insulating film is broken by the mechanical impact in the bonding step. In FIG. 1, each of a plurality of wiring layers 60 is formed by using an insulating film 61 having a low dielectric constant and a low density. Each of a plurality of metal wirings 62 is formed of, for example, a Cu layer buried in the surface region of each of the insulating films 61. Also, a bonding pad 63 formed of Cu is formed in the uppermost wiring layer 60 together with the metal wirings 62. Further, a passivation film 64 is formed on the uppermost wiring layer 60.
It should be noted that, if the insulating film 61 is formed of a material having a low dielectric constant and a low density, the insulating film 61 is broken in the corner portion below the bonding pad 63 by the mechanical impact in the bonding step of the bonding pad 63.
A second problem is that the semiconductor device is adversely affected by the gas or water generated from the insulating film in the process of forming the insulating film. Where the insulating film is formed by the coating of methyl polysiloxane, followed by baking the coated film, the coated film of methyl polysiloxane is crosslinked by the dehydration condensation reaction, with the result that a large amount of hydrogen and water are released in the process of forming the insulating film. It should be noted in this connection that, if a ferroelectric memory cell or a MIM (metal-insulator-metal) capacitor is formed in the LSI, the capacitor insulating films of these elements incur deterioration of the performance if heated under a hydrogen atmosphere.
FIGS. 2A to 2C are cross sectional views collectively showing the manufacturing process of a semiconductor device in which an MIM capacitor is formed in a single wiring layer forming a multi-wiring structure.
In the first step, formed is a wiring layer 60 in which a metal wiring 62 made of, for example, Cu is buried in a surface region of an insulating film 61 made of, for example, SOG (spin on glass), as shown in FIG. 2A. Then, a stopper insulating film 65 made of, for example, silicon nitride (SiN) is formed on the entire surface, followed by forming an MIM capacitor 66 on the stopper insulating film 65. The MIM capacitor 66 includes an upper electrode, a lower electrode, and a capacitor insulating film interposed between the upper electrode and the lower electrode and made of, for example, silicon nitride, tantalum oxide or titanium nitride.
In the next step, the entire surface is coated with, for example, methyl polysiloxane, followed by baking the coated film so as to form an insulating film 61 constituting the upper wiring layer, as shown in FIG. 2B. When the insulating film 61 is baked, a large amount of hydrogen (H) is released from the coated film of methyl polysiloxane. If the released hydrogen is heated, hydrogen is taken into the capacitor insulating film of the MIM capacitor 66 so as to deteriorate the performance.
Further, a metal wiring 62 made of, for example, Cu is formed in a manner to extend through the surface and the inner region of the insulating film 61, as shown in FIG. 2C.
Further, a third problem to be noted is that corrosion, erosion and peeling of another film is caused by the gas released from the insulating film. There is an insulating film having a high susceptibility to water and a high water permeability. Also, when it comes to an insulating film having a bond relatively low in its bonding energy, there is an unstable film that releases a gas under the temperature of about 350 to 400xc2x0 C. in the step of forming a multi-wiring structure. Need less to say, the characteristics of the insulating film itself that releases the gas are changed. In addition, the corrosion, erosion, and peeling of another insulating film are brought about by the released gas.
FIG. 3 shows the peeling of a film in the heating step in the case of using a film that is likely to release a gas as the insulating film 61 constituting a wiring layer. In FIG. 3, the stopper film 65 made of, for example, silicon nitride (SiN) is formed on the insulating film 61 constituting the wiring layer, followed by forming a new insulating film 61 on the stopper insulating film 65. If the new insulating film 61 is formed of a film that is likely to release a gas in the heating step for baking the insulating film 61, a gas is released from the insulating film 61. The released gas causes the peeling of the stopper film 65 in the portion where the bonding strength of the stopper film 65 with the underlying film is low, e.g., on the bonding pad 63.
As described above, it was customary in the past to use a single material for forming an insulating film included in the same wiring layer. Therefore, if it is intended to increase the operating speed by using an insulating film having a low dielectric constant, serious problems are generated in respect of the reliability that the insulating film is broken by a mechanical impact, that the element is adversely affected by the gas or water released from the insulating film, and that the corrosion, erosion, and the peeling of the film are brought about by the gas released from the insulating film.
According to an aspect of the present invention, there is provided a semiconductor device of a multi-wiring structure, comprises an electrode to which is applied a mechanical pressure; a first insulating film formed in a region where it is necessary to have a high mechanical strength and having the electrode formed therein; a second insulating film formed in the same layer as the layer of the first insulating film and formed in a region where a mechanical strength higher than that of the first insulating layer is not required; and a wiring layer formed on the surface of the second insulating film.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device having a multi-wiring structure, comprises coating a substrate with a film of an insulating material in which a crosslinking reaction or a foaming reaction is generated; subjecting the film of the insulating material to a heat treatment so as to bring about a crosslinking reaction or a foaming reaction, thereby forming a first insulating film on the substrate; selectively removing the first insulating film such that the first insulating film is selectively left unremoved on the substrate and is removed in the other region; and forming a second insulating film in the region where the first insulating film has been removed.